In order to improve the productivity of agricultural products, it is important to accurately determine the growth status of a crop so as to efficiently produce the crop. To this end, there is a known technique that irradiates a measurement light to a plant (e.g., crop (target plant)), receives the measurement light reflected on the crop, calculates a reflection rate at which the crop reflects the measurement light, and determines, based on the calculated reflection rate, a spectroscopy vegetation index such as a normalized difference vegetation index (NDVI) that is indicative of the growth status of the crop. Since the spectroscopy vegetation index is one of the parameters to determine the growth status of a plant, knowing the index makes a substantial contribution to the determination of the growth status of the crop.
In a conventional technique, a plant sensor device capable of appropriately receiving the light reflected on the crop to determine the spectroscopy vegetation index accurately and to appropriately determine the growth status of the crop has been taught (see JP 2012-247235 A, for example). The conventional plant sensor device can reduce the influence of a light volume component caused by a disturbance light and appropriately receive the reflected light from the crop. As a result, the conventional plant sensor device can accurately determine the spectroscopy vegetation index to appropriately determine the growth status of the crop.